Definition and Epidemiology

Syncope is defined as a temporary loss of consciousness and postural tone that is followed by spontaneous complete recovery and does not require resuscitation. Presyncope or near-syncope is a sensation of lightheadedness or faintness in which the patient senses that true syncope may be imminent but complete loss of consciousness never occurs.

The true incidence of syncope in the general population is not well known owing to differences in definition, under-reporting, and variations within age groups or special populations. There is a similar incidence for men and women until 70 years of age, after which there is a sharp increase in incidence that favors women.¹ Women are therefore twice as likely as men to experience syncope during their lifetime.² Syncope is more common in older patients than in other age groups and is often associated with falls and greater risk for adverse outcomes.³ The increase in syncopal events is likely to be related to the number of comorbidities and prescribed medications in this cohort.⁴ In addition, the physiologic changes of aging increase an elder’s risk for syncope.⁴

Pathophysiology

Syncope is a symptom of an underlying process or processes (Box 36-1). These processes result in syncope by one of two pathophysiologic mechanisms: deprivation of nutrients to the brain or deprivation of oxygen to the brain. Deprivation of nutrients most often results from decreased blood flow secondary to hypovolemia, cardiac outflow obstruction, cardiac arrhythmias, or neurovascular causes. Deprivation of oxygen is most often associated with hypoxia or anemia. It is important to distinguish true syncope from seizure disorders or other conditions that might result in altered levels of consciousness, such as iatrogenic syncope from medication therapy, drug or alcohol intoxication, concussions, amnesia, or metabolic causes such as hypoglycemia. Seizure-like activity may be present with syncope; this is secondary to generalized cerebral hypoxia.

There are three main classifications of syncope: (1) neurally mediated or reflex, (2) orthostatic hypotensive, and (3) cardiac.⁵ A review of the literature cites a fourth classification, neurogenic, which includes stroke.⁶ The most common cause of syncope is vasovagal; however, cardiac causes have an increased incidence of sudden death and must be evaluated early.⁵ In general, cardiac syncope is seen in older adults.⁶ The cardiac causes consist of two major categories: (1) mechanical or ventricular outflow obstructive processes and, more common, (2) arrhythmias. Possible mechanical or obstructive processes responsible for syncope include cerebrovascular disease, cardiac valvular disease, atrial myxoma, hypertrophic or obstructive cardiomyopathy, pulmonary hypertension, pulmonary embolism, pericardial disease or tamponade, acute myocardial infarction or ischemia, and possible prosthetic valve malfunction. Possible rhythm disturbances include sick sinus syndrome, atrioventricular conduction disturbances, supraventricular and ventricular tachycardias, long QT syndrome, and pacemaker system malfunction. Tachycardias can also trigger vasovagal syncope.⁵

Neurally mediated syncope is the most common type of syncope and is primarily seen in young adults.^2,6 Situational syncope, carotid sinus syncope, and others are also classified as neurally mediated. Although different in their provocation, these disorders share a reflex response that causes vasodilation, bradycardia, and paradoxical systemic hypotension, eventually leading to decreased blood flow to the brain.^5,6 In carotid sinus syncope, the trigger sites are thought to be peripheral receptors that respond to mechanical stimuli (such as neck stretching or tight collars); this type of syncope occurs most often in older adults.⁵

Finally, orthostatic stress can cause insufficient peripheral vasoconstriction, leading to syncope. Orthostatic hypotension is rare in patients younger than 40 years, yet is one of the most common causes of syncope in patients older than 70 years.⁶ Classic orthostatic hypotension is defined as a drop in systolic blood pressure (BP) of greater than 20 mm Hg or of diastolic BP of greater than 10 mm Hg within 3 minutes of transition from supine to standing.⁷ This can be triggered by blood loss, dehydration, or autonomic dysfunction. In the elderly population, syncope is often reported in the morning after taking medications.⁶ It is important to note that orthostatic stress can be present with both cardiac and neurally mediated syncope.⁵

Several miscellaneous causes of syncope are not easily classified into any of the previously mentioned categories. Hypoglycemia is a possible metabolic case of syncope and is usually found in individuals with diabetes who have taken too much of a particular hypoglycemic agent. Hyperventilation is another possible cause. Several psychiatric causes, including depression, hysteria, and panic attacks, may subsequently result in hyperventilation, which can lead to hypocapnia and cerebral vasoconstriction compounded by possible peripheral vasodilation.